Purified fiber



May 6, 1930. R, A GQRTNER El AL v PURIFIED FBER original Filed sept. 2, 1925 INVENTORS er and ATTORNEYS Patented -May 6, 1930 Ul. `lI'I l=`-Dr STATES-ll OFFICE ROSS GORTNER AND WALTER FRED HOFFMAN, OF ST. PAUL, MINNESOTA, AS i SIGNORS TO THE NORTHWEST PAPE-R COMPANY, OF CLOQUET,'MI NNESOTA,JA OOR- PORATION OIFv MINNESOTA PURIFIED FIBERV original application fned septeniber 2, 1925, serial No. 54,003. Divided and this application 'med January 30, 1926.

This invention relates to a method fdr purifying vegetable fibers, and Vspecifically Wood fibers and paper pulp. The paper pulp may V be cooked or given other chemical treatments,

such as bleaching, beforebeingtreated by our process. The wood fibers may be shredded or in' the form of chips. The object of this invention is to remove, by means of the electric current, a large portion of the ash present in vegetable bers andalso to remove organic coloring matter and impurities usualvly present in these materials. In addition to the object above disclosed,`.our invention further improves andpurifies such fibers so that they remain white and do no turn yellowr when subjected to the action of the atmosphere at elevatedtemperatures. A further object is to make a puried paper pulp which retainsvits original strength in comparison with the weakened pulps purified by drastic chemical treatments.

Our method is an improvement over the old methods of treating pulp and vegetablel fibers with alkalies and acids of various kinds contained therein.

'trolytic cell. Y

In the accompanying drawing there is illustrated, in diagrammatic form, al simple electrolyticcellwhich we preferably use to carry out our present'invention. In the drawing, 1 .is a suitable container, whichmay be constructed of Wood and made Water proof and tight in the usual manner. 1A typical cell which has been used successfully for' carrying out our invention but which may be modified in its dimensions and construction to suit conditions, has the following interior dimensions: 10 long, 6 wide, and 5" deep. Partitions, 4 and 5 are membranes which are permeable to the colloids which are present 1n 5u vegetable fibers, but which will retain the serial. No. '$4,863. v

fibers. We have found that ordinary 10 ounce canvas is suitable' for wood pul but we are not limited to this material. ny material which has the property of retaining the fibers to be purified but which allows the impurities to pass through and which is not readily destroyed by the contents or the action of the cell, is suitable'. If it is desirable to remove electrolytes and retain the colloids Vsuch as organic coloring matter and colloidal silica, a diaphragm impermeable-to colloids is used.

The diaphragms 4 and 5 are mounted across the cell so as to divide thecell into three compartments,6, 7 and 8; 7 being the anode and8 the cathode compartment, the direction of iow of the electric current being shown by the arrows. The compartments may be of equal capacity or the central compartment may be of much greater or smaller capacity than either of theanode and cathode compartments. In the anode and cathode com# partments suitable electrodes 2 and 3 are placed. The anode is preferably made of a graphite plate and in the specific cell described is about l1/g X 6 in area. Due to the presence of chlorides and sulphates, graphite is most suitable, due to the disintegrating action on other materials. The cathr form of a slurry which may contain about 5 to 10 kper cent lof pulp. If wood chips are used the Ycentral compartment is filled with them. The chips may be wet ordry. The cell is then filled with water to a suitable height to prevent overflowing. Heating the contents of the cell results in a )more rapid purification than is obtained at room tem-A perature, due to the greater mobility of the ions and colloids at the higher temperatures.

A suitable temperature is one Which ap-vl proaches the' boiling point of Water.

The electrodes are attached to any suitable source of direct current, and in the cell illustrated, which is about 10l long, a pressure of about 220 volts has been used to carry out the desired reaction for ordinary Wood pulp. This high pressure is desirable since the electrolyte consists of Water, the conducting ions being supplied by the impurities in the Water, the -ash in the fibers, and the salts remaining in the pulp after the Washing operations in thepulp mill. Chemicals may be added to the water. All electrolytes are decomposed and are removed from the anode and cathode compartments. The current, when chemicals are not added, has been less than one ampere for the cellillustrated, or less than 5 amperes per sq. ft. of the cross sectional area of the pulp in a plane parallel to the A electrodes. The higher thev ash content, the

greater the conductivity, thereby increasingy the current density. AThe current gradually drops and the liquid in one or both electrode The compartments, assumes aidark color. liquid in 4the anode compartment contains the anions such as Cl and S04 and. negatively charged colloids, while the cathode compartment liquid-contains positively charged col- 3* loids and the cathions such as calcium, so-

dium, and iron. The coloring matter in the Water ofthe electrode'compartments is probably due to the lignins and other organic' compounds-such as are contained in black liquor (not completely Washed out after the l cooking operation in the digestors) The impure liquids accumulated at the electrodes in the anode and/cathode compartments are preferably changed from time to time to 'prevent diffusion back into the pulp 0r vegetable fibers contained 'in the central compartment. Although the impure liquids maybe changed intermittently a continuous 'stream of fresh Water, which may be hot, can

be run into these compartments, as through pipes 9 and 10, and the impure Water removed by suitable overflow openings 11 and 12. ANot only may the impure Water be removed continuously, but the pulp may 'also be added to and removed from the central compartment to make the operation continuous.

The pulp is subjected to the electric Icurrent until it has reached the desired purity, or if. ahighly purified pulp is desired, the electrolysis is continued until the resistance ofthercell becomes so high ythat the current drops to almost zero, our measurements indicating below '.10 ampere per square footl of cross sectional area of the pulp. This low -conductivity indicates a high' degree ofpurity. The, finished pulp may be given a finall tion of the li'uid lin thecentral compartvyashin'g with pure Water. ,A

The pulp which is sub'ected to the electric current isvwhitened or bleached considerably,

if it has not been previousl bleached, and very appreciable amounts o coloring materialsmay be removed from pulps that have been chemically bleached. The whitening is no doubt due to the removal of the lignin or other-organic compounds. vBleaching may precede or follow the electrolytic treatment;

In addition the ash content ofthe liber or pulp has been decreased so that it may be below .05 per centand in some cases We have found it to be as low as .031 per cent. The

original ash in this pulp was 1.43 per cent.A

The electric current removes a large part of the inorganic impurities, especiallythose of the alkaline earths, alkalies, and heavy metals suchV as iron, but does not fully remove the l our treatment yields a dark liquor at the vanode and the color is improved. Kfhe pulp stays purewhite when heated at 60 to 70 degrees C. for hours. The pulp purified by our method thus has a special application to products which must not color when subjected to heat, which also indicates that they Will not turn yellow: with age.

Our method may also be used to Whiten unbleached fibers which may have been cooked and Washed to remove most of the chemicals and impurities. Authorities are generally of the opinion removed when subjected to the action of the 4electric: current in Water solution. This theory is borne out. by our results. Most of the inorganic impuritles are ionized and under the -influence of the electric current travel to the oppositely charged poles at either end .of thecell, resulting in the presence of chlorine and sulphate ions at the anode and the basic sodium, calciumand iron ions at the cathode; Colloidal silica particles of such sizethat; they show cataphoresis are removed by our process. Our experiments show'that silica is not entirely removed though it .possibly may be removed under certain condltions. `Such colored organic. compounds as l are colloidal will possess either a positive or a negative charge, the sign of the charge dependingupon the hydrogen ion concentraments, andun er the influence of the electrical current will travel either to the cathode compartment as the electrolysis proceeds.

The cellulose is normally negatively charged and tends to travel to the anode, but is stopped by thecanvas or other diaphragm and piles up against this diaphragm. If the cellulose becomes dense enough, Water may even pile up in the cathode chamber. Some 'l experiments show that it is possible to give the cellulose a positive charge so that it will travel toward the cathode chamber. In general our process removes electrolytes, and such colloids as may migrate by cataphoresis and which Willvpass through the membrane used. A membrane impermeable to colloids Will only remove electrolytes from the ber or pulp, and thus reduces the ash,

content by electro-dialysis.

While the hemi-cellulose and the degraded celluloses (Oxy-cellulose and hydro-cellulose) are considered to be adsorbed by the cellulose these apparently are not removed, or only partially removed, by the electrolytic action. It is ourbelief that these cellulose compounds and modications are so intimately connected with the pure cellulose as toapproach chemical combination so that they cannot be removed, or are only partially removed, under the conditions outlined. By electrolyzing the pulp in a Water suspension to which chemicals, such as acids or alkalis have been added, the results indicate that cellulose-like impurities are removed. Tests have shown that When a bleached pulp containing oxy-y cellulose is subjected to our process, the oxycellulose isstill present after this treatment, though the coloring compounds have been removed and the ash has decreased to less than .05 per cent.

While We have described a specic form of apparatus for carrying out our process,

' We do not Wish to be limited to this form but claim broadly the process of removing electrolytes and organic andA inorganic impurif ties, part or all of which may have been adsorbed on the cellulose bers, by passing an electric current through vegetable bers in Water suspension and removing such-impurities as are accumulated at jor near the electrodes. The diaphragms are used to keep the impurities away from again contaminating the pulp. Since the pulp usually has a tendencyto migrate toward the anode,.it may .be possi-ble to dispensewith the diaphragm at'the cathode, under'certain. conditions,and if thecellulose is positively charged, it may not be necessary to have a diaphragm at the* anode. Our invention'applieszto vegetable bers in general, and specifically to Wood bers, whether" in a gregates as in wood chips or disintegrate in the form of pulp,

and Wood pulp and paper pulp containing vegetable bers of various kinds, cooked or uncooked, and bleached or unbleached. This 'application is adivision of ourfco-pending application', Serial No. 54,003, led Septeml 2. Cellulose substantially free from ad' sorbed inorganic impurities except silica.

3. Paper pulp derived from Wood substantially free from inorganic impurities except silica.

4. Paper pulp substantially free from impurities except heini-cellulose, 'degraded cellulose and adsorbed mineral matter not excee'ding .05 percent and consisting essentially of silica. y

5. Vegetable ber substantially free from impurities except hemi-cellulose, degraded cellulose and adsorbed mineral matter con-4 sisting essentially of silica.

' 6. Cellulose derived from wood substantially' free from impurities except hemi-cellulose, degraded cellulose and adsorbed min# eral matter not exceeding .05 percent and.

consisting essentially of silica.

7. Apaper pulp containing less than .05 percent of ash, said ash consisting essentially of silica. l

8. A paper pulp which does not color onoo heating 'at 60 to-7O degrees C. for 100 hours, said pulp containing heini-cellulose, degraded cellulose, and less than .0,5 percent of ash consisting essentially of silica. ,l

9. A paper pulp consisting essentially of Wood bers,` substantially free from impurities excepthemi-cellulose, degraded cellulose and silica. f

10. A paper pulp consisting essentially of `wood bers, substantially free from impuriof `vvood bers, containing an ash consisting essentially of silica.

13'. A paper pulp consisting essentially of Wood bers, containing less than .05 percent of ash, said ash consisting essentially of silica.

14 Vegetable ber' consisting essentially of .Woodbers' which does not color on heat-l ing at or 70 degrees C. Jfor 100 hours,said ber-'containing hemi-cellulose, degraded cellulose, andless than .05 percent of ash con-A sisting essentially of silica.

15. A'paper pulpvvhi'ch docs not color on l heating at 60-7 0 C. for 100 hours, said pulp containing less than .05% of ash, consisting essentially of silica. D

16. Ve etable liber consisting essentially' of wood ber, which does not color on vheating at 60-70 C. for 100 hours, said fiber 'containing less than" .05% ash consisting essentially of silica.

In testimony whereof we affix our signatures.

ROSS AIKEN GORTNER. WALTER FREDY HOFFMAN. 

